Topband: Spurious signals

[Tom Rauch]

 > Interesting...
 > Shouldn't using a properly adjusted antenna tuner let the amplifier
 > see a 50 ohm load impedance?

Only on or near the operating frequency.

If someone forwards me a copy of a PIN switched ETO amplifier, I'll
put the diagram up on a web page and show the feedback path and how
to cure it.

If you have a PIN switched Ameritron, there was a factory mod made
back around 1985 to cure that problem. I'll put something up on my
web pages about that also.

The solid state amp is easier to understand. In that case, when you
add a T network you open the load at low frequencies (remember the T
is a high-pass and looks like an open circuit below 160 meters).

When the amplifier is open-terminated at low frequencies, gain (at
low frequencies) skyrockets and the amp oscillates. The low-frequency
oscillation modulates and mixes with the 160 signal, and the result
is hissy buzzy spurs all over the place.

I fight that problem, and I'm sure others do, every time I design a
solid-state PA. Transistors are a nasty problem at low frequencies,
because gain becomes so high. It's nearly impossible to make them
unconditionally stable, and the worse possible thing is an open load
at low frequencies.

What I'm trying to do is identify all of the amps or rigs that do
this, get schematics, and reverse engineer them so users can fix
them. One simple cure that often works is to add a 1/4 wl shorted
stub between the amp and the tuner.73, Tom W8JI
W8JI@contesting.com